Process for producing high wet strength paper



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F; Walker. ummifl iy du Pont de Nemours' a Gomm 'wu I y I Del. acorporation of No Drawing.

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November M1 N; 365,984

scum. (o 11pm) c This invention, relates to the reatment of papers and,more particularly, to tne imparting of certain desired surface and bodyproperties to said paper by said treatment.

Papers of high wet strength fall in the following categories asdesignated by theirmode of preparation:

1. Parchments, fish papers, etc., are made by treating water leaf websof cellulosic fibers with such as casein, glue ,or vegetable proteinwith formaldehyde to produces limited degree of water resistance. Theproteins are unstable and f they impart a yellow cast to the coating.

This invention has as its object the production of a high wet'strengthpaper. Another object is to produce such paper by a method that willoperate at speeds .of at least 500 ft. per min.

Another object is to produce a high wet strength paper free from odorand injurious substances. Still another object is to produce a paperhaving a high wet strength and which also has good printing properties,is resistant to finger-marking, and is capable of being washed withwater.

Another object is to prepare a high wet strength paper which has a highcolor. Other objects will of paper-making fibers before or after webformation with specific after-treatment to convert these adhesivesubstancesinto the waterinsoiuble form. The viscose treatment requiresconsiderable washing at time-consuming speeds. The glue treatmentrequires formaldehyde to insolubilize the glue, the product is odorousand it must contain'large ratios of glycerol or other softeners.Urea-formaldehyde treatedpaper is very stiff. Phenol-formaldehydetreated paper is very stiff, odorous and colored.

3. Such papers may be formed by treatment of webs of paper-making fiberswith formaldehyde and mineral acid catalysts and heating above 110 C. toabsolute dryness. The product is degraded considerably by dryingcompletely at such high temperatures in the presence of mineral acidsand it gives off free formaldehyde whenever moistened, so it constitutesa health hazard.

4. These papers may be formed by flash heating a web of paper-makingfibers with or with-. out a glue size at 400 to 450 C. The paper isdegraded somewhat by the high temperature of heating.

5. These papers may be prepared by impregnation of dry paper webs withsolutions of resins or water-insoluble cellulose derivatives in organicsolvents and drying. The raw materials are costly due in part to the useof organic solvents and in part to an extra processing. v

6. These papers may be prepared by application of pigmented coatingcompositions to one or both sides of the paper web using adhesives,

beapparent from the the invention.

These objects are accomplished by applying to a web of paper-makingfibers polyvinyl alcohol and a water soluble N-methylol aliphaticnitrogen-containing compound preferably in aqueous solution'having a pHin excess of 5.5 and heating said treated paper to such a degree as tocause an insolubilizing reaction between the polyvinyl followingdescription of alcohol and the N-methylol compound. This insolubilizingreaction is best carried out in the presence of a catalyst such as anacid, preferably,

however, in the presence of an ammonium salt of a mineral acid.

The aqueous solution of polyvinyl alcohol is mixed with thewater-soluble N-methylol aliphatic nitrogen-containing compound,preferably a methylolsmelamine and an ammonium salt of a mineral acid inconcentrations that give workable viscosities, is applied to one or bothsides of the paper simultaneously, is passed through a drier systemwhere both surfaces are in contact with metal surfaces that are at atemperature above C. until dried to a moisture content of about 4 to 20%or until the wet strength is at least 50% greater than that of the same,

without departing from' thespirit and scope of this invention.

Example I A web of paper is formed from a mixture of 70 parts of groundwood and 30 parts of unbleached sulfite flbers into a sheet of 34 lbs.for a 24 x 36 x 500 ream. The dry sheet is sprayed on one side with asolution comprising 5.0 parts polyvinyl alcohol, 1.0 part trimethylolmelamine and 94 parts water in sufllcient amount to add 1.0 partpolyvinyl alcohol to 100 parts dry fiber weight. An aqueous solution ofammonium chloride is then sprayed on the sheet in suflicient amount toadd 10.7 parts ammonium chloride. The sheet is then pressed against aheated, smooth, metal cylinder by means of a canvas belt for 30 seconds.The tensile strength of a strip 0.5 by 5.0" in wet state is 3.51 lbs.compared to 0.42 lb. for a strip of the same paper that is treated withwater only before drying. The polyvinyl alcohol is of the hotwater-soluble type with an intrinsic viscosity of 23 centipoises in 4%aqueous solution at 25 C. and with a saponification value of 6. Themethylol melamine is prepared by adding 1 mol anhydrous melamine to 3mols formaldehyde as a neutral 37% solution in water and methanol,heating at 80 C. for 30 minutes, pouring into enough hot water to make a5% solution and cooling.

Example 11 A 50-lb. sheet (ream=24 x 36 x 500) of white, sized,clay-filled paper is coated on the wire side with a clay slurry to theextent of 4 lbs. solids per ream. It is dried bypassing the coated sideover the smooth surface of heated metal cylinders for 30 seconds using aregular drying canvas to press'the sheet against/the cylinders which areheated by lbs. gage steam pressure. The product is super-calendered to ahigh gloss. It gives a minimum coating pick of 6 and a minimum fiberpick of 9 and it withstands more than 4000 wet rubs without losingpigment from the surface.

The coating composition consists of 100 parts 2* high grade coatingclay, 5.0 parts polyvinyl alcomonium chloride and 196 parts water. Thesame sheet coated in the same manner with a slurry consisting of 100parts of clay, 5.0 parts polyvinyl alcohol, and 202 parts water losespigment on one wet rub. The polyvinyl alcohol has an intrinsic viscosityof 22 centipoises in 4% aqueous solution at 25 C. and has asaponification value of 11. The wet rub test is made by applying 10drops of distilled water to the coated surface and rubbing with theforefinger in one inch strokes. Water is added dropwise as needed tokeep the area wet. The white pigment is readily recognized when loosenedfrom the paper. The same gage steam pressure.

these cylinders is seconds and the moisture ples. minutes are treatedwith an additional 0.2 cc.

sheet coated in the same manner with a slurry consisting of 100 parts ofclay, 15 parts casein,

37% solution of formaldehyde 0.6 part, and water 214 parts; and driedunder the same conditions gives a minimum coating pick of 6, a maximumfiber pick of 8, and 3400 wet rubs.

Example III A water leaf web of 30 lbs. weight for 24 x 36 x 500 ream ifformed from a Mitscherlich grade of bleached sulfite fiber that isbeaten to the condition to form a greaseproof paper with density of 17lbs. per mil per ream. After passing over enough drying cylinders toreduce moisture content to the sheet is passed between tub-size rollswhere both sides are treated with a solution of polyvinyl alcohol 3.0parts, dimethylol ammonium sulfamate.0.5 part, ammonium nitrate 4.0parts, and water 92.5 parts; and squeezed sufficientiy to leave.2%polyvinyl alcohol based on a Wet tensile strength of 6.8 lbs.

the dry fiber weight. The polyvinyl alcohol has an intrinsic viscosityof 17 centipoises in a 4% aqueous solution at 25 C. and has asaponification value of 7.2. The sheet then passes over more dryingcylinders that are heated with 10 lbs. The time of passage over contentof the sheet on emerging is 7%. The sheet is then' moistened further bypassing through a solution containing glycerol 3.0 parts, invert sugar9.0 parts, and water 88 parts with enough solution absorbed to add 4.0parts total softener based on dry sheet weight, is allowed to stand inloose roll form 24 hrs. and is supercalendered with alternate rollsheated by lbs. gage steam pressure. The product is a smooth, densehighly polished sheet of glassine paper'with a resistance to turpentinegreater than 8 hrs. and The turpentine resistance and wet strength ofsame sheet to which the glycerol-invert sugar mixtureis applied to theextent of 8.0 parts instead of the polyvinyl alcohol composition is 3hrs. and 0.6 lb., respectively. The turpentine resistance test is madeas follows: Cut 16 samples 2" sq. each, from representative areas ofeach sample of paper; these are'given a sharp bend a quarter inch fromeach end to facilitate handling. They are then arranged, on a largesheet ofsmooth white paper which serves as a tell-tale. One-inch squaresof blotting paper (kerosene absorption 350%) 0.050 thick are placed oneach test piece. Then 0.4 cc. of anhydrous turpentine, colored with ared dye (2.0 parts oil soluble red to 98 parts turpentine), is appliedto the blotting paper square and a 1" cube of brass is placed promptlyonto this square of blotting paper. Test sheets with blotting paper andbrass cubes in position are pulled along the surface of the tell-talepaper to new positions at intervals in order to determine time in whichthe tell tale sheet is marked by the colored turpentine. The turpentineresistance is expressed in minutes as a mean for all 16 sam- Samplesthat show no penetration at 480 colored turpentine and this is repeateduntil completion of test. Eight strips of 0.5 x 5.1 are cut across thegrain and the other set with the grain. They are submerged in distilledwater 15 minutes at 25 C., excess water is removed from the surface ofeach strip just before testing by means of blotting paper. The tensilestrength is determined on an inclined plane type of Scott tensile testerwhich records strength and stretch auto-- matically at intervals ofapproximately 0.01 lb.

Example IV A 40-lb. sheet of white, sized, clay-filled paper is coatedon both sides at once with 4 lbs. solids per side per ream by passingthe dry sheet be-- tween rubber covered rolls and drying by passing oversteam heated cylinders that are treated with 20 lbs. gage steampressure. The time of heating is 30 seconds. The coating compositionconsists of 100 partsof chalk, 7 parts polyvinyl alcohol, 1.2 partsdimethylol thiourea, 5.0 parts ammonium chloride, and 138 parts ofwater. After calendering it has a minimum coating pick on the wire sideof 6, a minimum fiber pick of 8 and wet rub resistance of 2176. This isin contrast to a coating pick of 5, fiber pick of T1, and wet rubresistance of 0 for the same coating in which the dimethylol thioureaand ammonium chloride are omitted. The polyvinyl alcohol has anintrinsic viscosity of 18 centipoises in a 4% aqueous solution at 25 C.and a saponification value of 7.0. The dimethylol thiourea is preparedby adding 1.0 mol dry thiourea to 2.2 mols of a neutral formaldehydesolution of 37'??- strength, maintaining the reaction mixture at 40 to50 C. for 1 hr. and then at room temperature for 12 hrs., and thenadding to this solution gradually a suflicient amount of boiling waterto make a 5% solution and cooling to room temperature.

Example V a system of cylindrical driers heated to 5 lbs.

gage steam pressure with total heating time of 45 seconds. It is thencalendered to develop gloss and smoothness. The coating compositionconsists of calcium sulfite 100 parts, polyvinyl alcohol 8 parts,monomethylol adipamide 0.6 part, ammonium bromide 2.0 parts, and water110 parts. The polyvinyl alcohol has an intrinsic viscosity of 65 in a4% aqueous solution at 25 C. and has a saponification value or 245. Thecoating after calendering has a coating pick of 6, a fiber pick of 9 andwet rub resistance of 1570 in comparison to a coating pick of 6, a fiberpick of 8 and wet rub resistance of 0 for the same coating from whichthe monomethylol adipamide and ammonium bromide are omitted. I

Example VI A web of paper is formed from a mixture of ground wood '70parts and unbleached sulfite 30 parts into a sheet of 34 lbs. for 24" x36 x 500 ream. The dry sheet is sprayed on one side with an aqueoussolution comprising polyvinyl alcohol 1.0 part, trimethylol melamine 0.2part and water 98.8 parts in sufficient amount to add 1 part polyvinylalcohol to 100 parts bone dry weight fiber. The sheet is'then pressedagainst a heated, smooth, metal cylinder by means of a canvas felt for60 seconds. The tensile strength of a strip 0.5" by 5.0" in the wetstate is 165 grams. (For this test strips are submerged in distilledwater at 25 C. for 15 minutes, excess water is removed by blotting andtensile strength is determined.) The polyvinyl alcohol is of the hotwater soluble type with a specific viscosity of 23 centipoises and asaponification value of 6. The trimethylol melamine is prepared byadding 1 mol dry melamine to 3 mols formaldehyde as a neutral 37%solution in water and methanol, heating at 80 C. for 15 minutes, lettingstand for 8 hours at room temperature, adding water enough to make asolution, heating to 80 C. just long enough to effect solution andpouring enough cold water into this hot solution to make a 5% solution.For comparison with the same paper treated with equal amounts of otheraqueous solutions and dried under the same conditions the followingtable is given:

sent percent of the ingredients absorbed in the paper on the basis ofbone dry weight of fibers.

Example VII pressing the coated side against the smooth surface of aheated metal cylinder for 30 seconds using a regular drying canvas topress the sheet against the cylinder which is heated by 20 lbs. gagesteam pressure. The product is supercalendered to a high gloss. It givesa minimum coating pick of 5 and a minimum fiber pick of 8 and itwithstands more than 4,000 wet rubs-by hand without losing pigment fromthe surface. The coating composition consists of parts of a high gradecoating clay, 5.0 parts polyvinyl alcohol, 2.1 parts trimethylolmelamine, 1.0 part ammonium nitrate, and 200 parts water. The pH of thiscoating slurry is 6.4. The same sheet coated in the same manner with aslurry consisting of 100 parts of clay, 5.0 parts polyvinyl alcohol, and202 parts water loses pigment on one wet rub. The polyvinyl alcohol hasa specific viscosity of 31 centipoises and a saponification value of 15.

The same sheet coated in-the same manner with a slurry consisting of 100parts clay, 15 parts casein, 0.6 part formaldehyde, and 214 parts waterand dried under the same conditions gives a minimum coating pick of 6,and a minimum fiber pick 01' 8, and 3,400 wet rubs.

Example VIII A water leaf web of 30 lbs. weight for 24 x 36 x 500 reamis formed from a Mitscherlich grade of bleached sulflte fibers that arebeaten to the condition to form a grease-proof paper with density of 1'7lbs. per mil per ream. After passing over enough drying cylinders toreduce moisture content to 45% the sheet is passed between tub-sizerolls where both sides are treated with a solution or 3 parts polyvinylalcohol,

0.6 part trimethylol melamine, 0.4 part ammonium sulfate and 96 partswater and squeezed sufficiently to leave 2% polyvinyl alcohol based onthe dry fiber weight. The polyvinyl alcohol has an intrinsic viscosityof 12 centipoises in 4% aqueous solution at 25 C. and a saponificationvalue of 5.0. The sheet then passes over more drying cylinders that areheated with 10 lbs. gage steam pressure. Time of passage over thesecylinders is 30 seconds and the moisture content of the sheet onemerging is 6%. The sheet is then moistened by passing through asolution containing. 1.0 part glycerol, 3.0 parts invert sugar, and 96parts water with enough solution absorbed to add 4.0 parts totalsoftener based on dry sheet weight, is allowed to stand in loose rollform for 24 hours, and is supercalendered. The product is a smooth,dense, highly polished sheet of glassine paper with a turpentineresistance greater than 8 hours and a wet tensile strength of 7.2 lbs.The turpentine resistance and wet strength of same sheet to which theglycerol-invert sugar mixture is applied to the extent of 8.0 partsinstead of the polyvinyl alcohol composition is 3 hours and 0.6 lb.,respectively.

E'zample IX A 40 lb. sheet of white, sized, unfilled paper is coated onboth sides at onc with 4 lbs. solids per side per ream by passing thedry sheet between rubber covered rolls and drying by PL sing over steamheated cylinders that are heated with lbs. gage steam pressure. The timeof heating is seconds. The coating composition consists of 100 parts ofchalk, 7.0 partspolyvinyl alcohol, 3.0 parts methylol melamine, 4.0parts ammonium chloride, and 137 parts water. It has .a pH of 9.8..After calendering it has a minimum coating pick on the wire side of 5, aminimum fiber pick of 8, and wet rub-resistance of 3,050. This is incontrast to a coating pick of 5, fiber pick of 7, and wet rub resistanceof D for the same coating, in which the methylol-melamine and ammoniumchloride are omitted. The P ly inyl alcohol has an intrinsic viscosityof 18 centipoises in 4% aqueous solution at 25 C. and has asaponification value of 7.0. The methylolmelamine is prepared'by adding1.0 mol dry.

melamine to 4.5 mols of a neutral formaldehyde solution of strength,heating in a water bath under reflux 30 minutes, cooling rapidly andthen adding this solution gradually to sufiicient boiling water to makea 3% solution, and cooling this solution as soon as transparency ofsolution is eiiected. i

Example X v A sheet of cardboard weighing 240 lbs. per ream is coated onone side with 12 lbs. solids per ream by passing the dry sheet through abrush coater system and drying by passing over a systern of cylindricaldriers heated to.5 lbs. steam 1 pressure with total heating time ofseconds.

It is then calendered to develop gloss and smoothness. The coatingcomposition consists of 100 parts of calcium sulfite, 8.0 partspolyvinyl alcohol, 0.3 part trimethylol-melamine, 2.0 parts ammoniumbromide, and 112 parts water, and has a pH of 8.1. The polyvinyl alcoholhas an intrinsic viscosity of 65 in a 4% aqueous solution at 25 C. andhas a saponification value of 245. The coating after calendering has acoating pick of '7, a fiber pick of 10, and wet rub resistance of 1920in comparison to a coating pick of 6, a fiber pick of 8, and wet rubresistance of 0 for the same coating from which the trimethylol melamineand ammonium chloride are omitted. The trimethylol melamine is preparedby the method described in Example I.

Although the preferred ratio of polyvinyl alcohol to fibrous ingredientsfor towel or tissue paper is 1.0 part to 99 parts fiber on bone dryweight basis, smaller and larger ratios of polyvinyl alcohol are usefulfor certain special requirements. Ratios of polyvinyl alcohol as low as0.3 part impart appreciable wet strengths to such papers. Where veryhigh resistanceto grease is required or where extremely high wetstrength is required ratios of polyvinyl alcohol up to 12.0 parts areuseful. The ratio of trimethylol melamine to polyvinyl a cohol may varyfrom a minimum of 0.1 part for 5.0 parts polyvinyl alcohol to maximum of2.0 parts when an acid catalyst is used.

When no catalyst, acid or neutral. is used the ratio of'trimethylolmelamine to polyvinyl alcohol may vary from a minimum of 0.5 part for5.0 parts of polyvinyl alcohol to a maximum of 5.0 parts. The acid maybe used in quantities to produce a pH as low as 5.5. Use of more acidwill cause undue degradation of the paper.

Any paper web may be used which is made in single or multiple layers andthe fiber composition may be the same for all layers, or the surfacelayers may be different, such as a cardboard with body or unbleached andsurface of such as difierent pigments and different pig-.

ment content in body and surface layers; or the surfaces of the web maybe presized with various materials such as starches or glues. The webmight also be sized with rosin or other emulsion sizes or it might beinthe water leaf condition.

The webs of paper are prepared by any of the well known methodsofpreparing webs from. fibers of paper-making length, such as on acylinder mold, a Fourdrinier, or a Harper Fourdrinier; and they may beprepared in any thickness of single or multiple layers and at densitiesfrom about 20 lbs. per mil to 4 lbs. per mil for a 24 x 36 x 500 ream.Theymay be made from fibers alone in highly beaten or slightly beatenform or they may contain mineral fillers, sizes,

waxes, etc. x

The fibers may be made from wood by the sulfate, sulfite, soda or groundwood method in bleached or unbleached form. They may also be made fromcotton rag, linen rag, flax, ramie, jute, straw, pineapple, Musatextz'lis, or banana in bleached or unbleached form.

The sheets may. vary in density from that of glassine to the most bulkybook paper or from 20 lbs. to 5 lbs. per mil for a 24 x 36 x 500 ream.

In addition to the compounds containing methylol groups given inexamples, other substances containing methylol groups may be used, suchas those made by reacting formaldehyde, paraformaldehyde or trioxane, orother com.- pounds yielding formaldehyde, with amines or amides orderivatives of amines or amides, all of which should be at leastslightly soluble in water. Typical examples of aliphatic amines, amides,etc., that will form useful insolubilizer for polyvinyl alcohol aremelamine, urea, thiourea, biuret, formamide, acetamide, guanyl ureasulfate, monobutyl urea, hexamethylenediamine, hexamethylenetriaminesulfamate, ethanolamine, diethanolamine, monomethylamine,monoethylamine, ethyl formamide, ethyl acetamide, and derivatives ofthese. Ammonium, sodium or calcium sulfamate may also be used. Thepreferred method of preparation consists in neutralizing the acidity ofthe formaldehyde solution, neutralizing the reactive nitrogen-containingcompound if it is strongly alkaline, and then mixing the formaldehydewith. the other reactants as concentrated aqueous solution in propermolal ratios at a slightly elevated temperature. In some cases theconcentrated forms of formaldehyde, such as paraformaldehyde ortrioxane, are preferred in order to obtain a high yield.

The preferred methylol melamine is prepared by adding 1 mol of2,4,6-triamino-1,3,5-triazine (melamine) to 3 mols formaldehyde as a 35%solution of formaldehyde in water and methanol which has been adjustedto a pH of 3.5 to 6.2, heating at C. for 15 minutes and cooling rapidlyto stop the reaction by diluting with cold water until a clear solutionis obtained of a concentration below 6% methylol-melamine. The productmay be used directly or before the dilution stage it may be dried at lowtemperature to a white powder. The white powder is nonhygroscopic anddissolves in water at 80 C. to make a clear solution that remains'clearwhen .cooled at concentrations below 6%. Slight alterations in thismethod may be used to obtain more complete reaction without renderingthe product insoluble at concentrations below 6%,

such as by cooling rapidly without dilution after the minute heatingperiod, letting stand at room temperature 12 hours, diluting with hotwater and cooling.

If the pH of the formaldehyde solution is below 3.5 the reaction is notso readily controlled and products maybe obtained that are insoluble inwater. 11' the pH of the formaldehyde solution is above 7.0 the producttends to be insoluble at concentrations below 6% unless acid is added toa pH of not greater than 3.0. Or it the temperature of heating isincreased much beyond 80 C. or if the time of heating is extended muchbeyond 15 minutes, such as to minutes, the product is hygroscopic, formsclear solutions at 25 to concentrations and precipitates on furtherdilution in cold or hot water. The preferred product is soluble in waterat concen-- trations below 6%, is readily blended with other ingredientsin the paper-treating compositions, is more eflicient than the typewhich precipitates on dilution in water, and can be handled with greaterease.

Methylol-melamine compounds are also useful that contain lower or higherratios of methylol groups, such as monomethylol melamine and up to asmany .methylol groups as can be added without making a water-insolubleproduct. At least a slight degree of water solubility is essential toinsure the reaction between the methylol melamine and the polyvinylalcohol.

The dimethylolurea is prepared by treating 2 mole formaldehyde as aneutral 37% solution in water with 1 mol urea to form a water whiteproduct that is freely soluble and stable as a 10% solution for severalweeks at room temperature.

less in amount than 4% produces a brittle product and destroys thequalities desired and obtained by the use of this invention. The time ofheating should be such that there is an insolubilizing reaction betweenthe starch and the water-soluble N-methylol aliphatic nitrogencontainingcompound. For successful operation at the drying step there should bepresent in the paper at the start of the heating period an amount ofwater equivalent to at least 30% of the total solids. Where it isinconvenient to heat the paper to a temperature of C. or above for asuiiicient time to dry the sheet, the sheet may be dried at a lowertemperature, then remoistened with'enough water to make at least 30%moisture (based on sheet weight) and then heated at 65 C. or above untilthe moisture content is about 5%.

Although the preferred ratio of polyvinyl alcohol to fibrous ingredientsfor towel or tissue paper is 1.0 part to 99 parts fiber on a dry weightbasis, smaller ratios and larger ratios of polyvinyl alcohol are usefulfor certain special requirements. Ratios of polyvinyl alcohol as low as0.1 part impart appreciable wet strength to such papers. Where a veryhigh resistance to grease is required or where extremely high wetstrength'is required, ratios of polyvinyl alcohol up to 12 parts per 100parts of fibers are useful. The preferred ratio for grease-proof papersis polyvinyl alcohol 2.0 parts per 100.

The range of ratios for polyvinyl alcohol to pigment in coatingcompositions may vary from 3.0

The polyvimrl alcohol should be of the watersoluble type with a specificviscosity as a 4% solution at 25 C. of 1 to 100 centipoises and asaponiiication value low enough to insure solubility in water, e. g.,below 350. They are tough, water-white, resimlike materials which areusually obtained by the hydrolysis of polyvinyl esters such as polvinylacetate. They are soluble to insoluble in cold water, but all of themare soluble in hot water and remain dissolved when the solution iscooled. The term polyvinyl alcohol" is used herein and in appendedclaims to designate generically pure polyvinyl alcohol and also suchpartial derivatives thereof that contain a sufllcient number of freehydroxyl groups to render the composition soluble in water.

Heating to effect insolubilization is best accomplished during thedrying step. The freshly treated web is dried at elevated temperature toa moisture content in the sheet of' 4 to 20% or until the wet strengthis at least 50% greater than that of the same sheet material treatedunder the same conditions with the same amount of polyvinyl alcohol butwithout the addition of th N-methylol aliphatic nitrogen containingcompound and the ammonium salt. In this drying step any temperatureabove 65 C. has been found to be operative for producing theinsolubilizing reaction. Theupper temperature limits' from the point ofview of operativeness depends upon the degradation temperature of thecompounds in the treating composition. Temperatures between 100 and 125C. are, however, pre-,' ferred as steam at temperatures inexcess of 1250-1a relatively expensive. It is essential that the heating step becarried out in the presence of moisture and that the moisture content ofthe paper at the termination of the heating be not less than 4%. Heatingto a moisture content of parts to 15.0 parts polyvinyl alcohol for partspigment. The preferred ratio of polyvinyl alcohol to pigment in coatingcompositions varies with the pigment and paper base, but falls betweenpolyvinyl alcohol 3 to 8 parts to 100 parts pigment.

The ratio of N-methylol compound to polyvinyl alcohol may vary from aminimum of 2 parts to 100 parts of polyvinyl alcohol to a maximum of 40parts to 100 parts of polyv nyl alcohol. It is preferred, however, tooperate within the ratio of 52100 to 151100.

The ratio of ammonium salt of a mineral acid to N-methylol compound mayvary from a minimum of 0.3 part of ammonium salt to a maximum of 12parts of ammonium salt for 1 part of N- methylol compounds. Thepreferredratio of ammonium salt to N-methylol compound is in the range of 1 to 5part of salt per 1 part of N-methylol compound.

The ammonium salts of mineral acids, such as hydrochloric, hydrobromic,nitric, sulfurous, sulfuric, persulfuric and phosphoric acids areoperative in this process; e. g., ammonium chloride, ammonium bromide,ammonium nitrate, ammonium sulfate, ammonium sulflte, ammoniumpersulfate; ammonium phosphate. The ammonium salts of a few strongorganic acids can also be used. The 2% aqueous solution of the aboveammonium salts should have a pH not lower than tinson laboratory coater.The amount applied.

was equivalent to 12.5 lbs. of solids per ream as catalyst.

'I (3,300 sq. ft. of paper). The coated sheets were air dried for from35 to 60 minutes, then calendered and heated with the coated side incontact with the metal surface at a temperature of 100 C. for 30seconds. The water resistance of the coating was then determined by theusual wet rub method. In all of the tests the polyvinyl alcohol employedwas one having an intrinsic viscosity of 25 centipoises in 4% aqueoussolution at 20? C.

Parts by weight Wet rubs Test #1:

Polyvinyl alcohol. Clay (kaolin).

Water Sodium silicate-42 Be. aqueous solution Test #2:

Polyvinyl alcohol Clay (kaolin) Trimethylol melamine Sodium silicate-42Be. solution.

Test #3:

Composition the same as in test #2,

but made acid to methyl red indicator by the addition of l-N HClcolution 8 100 20l l to 2.

More than 300 More than 300.

Trimethylol melamine Sodium silicate-42 Be. solu Water 203. Test Same as#4 but the mixture made acid to methyl red with l-N HCL For promotingthe insolubilizing reaction of a methylol melamine on polyvinyl alcohol,an am.-

monium salt of'a mineral acid has advantage over a mineral acid inrespect to scope of use and degree of insolubility that can be obtainedwithin limits of acidity which can be tolerated in commercial practice.

In order to develop a high degree of insolubility a pH as low as 3.0must b obtained. This and they function in the presence of alkalinepigments such as satin white and commercial silk.

The pH range of the clear coatings is preferably maintained between 6.1and 7.0 to obtain maximum insolubility and to prevent subsequentdegradation of the paper due to acidity.

The pH range of the pigmented compositions is determined in some cases(with chalk, satin white, etc.) by the nature of the pigment. .It can beas high as 12.5 with ammonium chloride The preferred pH range ofpigmented compositions is 6.1 to 11.5.

"cation of the treating compositions.

Softeners may be used in the treating compositions or may be applied tothe paper after appli- These softeners make the product more pliable.Examples of ;,such softeners include formamide, ethanol formamide,ethanol acetamide, glycerol, glycols, invert sugars,.certain hygroscopicsalts, such as sodium lactate, and higher fat alcohol sulfates. Otheringredients such as anti-foams can also be used.

The products are useful as printing papers,

- foodwraps, bags, liners for boxes, cartons, and

uses for packaging materials that require resistance to greases, fats oroils.

It is apparent that many widely difierent embodiments of thi inventionmay be made without departing from the spirit and scope thereof andtherefore it is not intended to be limited except as indicated in theappended claims.

We claim:

1. The process which comprises treating a web of paper-making fiberswith an aqueous solution of polyvinyl alcohol and a water-soluble N-methylol aliphatic compound said solution having a pH within the rangeof 6.1 to 1245, and then heating said treated paper web between 30 and60 seconds to such a degree as to cause an insolubilizing reactionbetween the polyviny alcohol and the N-methylol compound.

2. A process in accordance with claim 1 characterized in that thepolyvinylalcohol has an intrinsic viscosity as a 4% solution in water at25 C. of from 1 to 100 centipoises and has a saponification number below350.

3. The process which comprises treating a web of paper-making fiberswith an aqueous solution of'polyvinyl alcohol and an ammonium salt of amineral acid and a water-soluble N-methylol aliphatic compound saidsolution having a pH within therange of 6.1 to 12.5 under conditionsthat will leave at least 30% moisture in said web,

-that said paper web will contain less than 4% moisture.

4. The process in accordance with claim 1 characterized in that thewater-soluble N-methylol aliphatic compound is a methylol melamine.

5. The process in accordance with claim 3 characterized in that thewater-soluble N-methylol aliphatic compound is a methylol-melamine.

6. The process in accordance with claim 1 characterized in that thewater-soluble N-met-hylol aliphatic compound is trimethylol-melamine.

7. The process in accordance with claim I characterized in that thewater-soluble N-methylol aliphatic compound is dimethylol biuret.

8. The process in accordance with claim 1 characterized in that thewater-soluble N-methylol aliphatic compound is dimethylol ammoniumsulfamate.

GEORGE L. SCHWARTZ. JOSEPH F. WALKER.

I CERTIFICATE OF comcwron. Patent No. ,522,888. June 29, 1915..

of the above numbered patent requiring correction es follows: Page 2,first column, line 65, for 911 formed" read --is formed,- page 6, firetcolunin, line 57, for "silk" read -'chalk--; aild that the said-LettersPatent Henry van Arsdale, (Seal) Acting Commissioner of Patents.

